Ammonia is a widely used chemical with many applications. One specific application is as reductant for selective catalytic reduction (SCR) of NOx in exhaust gas from combustion processes.
For most applications, and in particular in automotive applications, the storage of ammonia in the form of a pressurized liquid in a vessel is too hazardous. Urea is a safe, but an indirect and impractical method for mobile transport of ammonia since it requires urea to be transformed into ammonia by a complex process involving spray, evaporation, thermolysis and hydrolysis ((NH2)2CO+H2O→2 NH3+CO2), which is difficult to achieve under driving conditions with low engine load or cold weather.
A storage method involving ad- or absorption of molecular ammonia in a solid can circumvent the safety hazard of pressurized liquid ammonia and eliminate the problem with conversion of a liquid reductant.
Metal ammine salts are ammonia absorbing and desorbing materials, which can be used as solid storage media for ammonia (see, e.g. WO 2006/012903 A2), which in turn, as mentioned above, may be used as the reductant in selective catalytic reduction to reduce NOx emissions.
WO 2008/077652 A2 discloses a method and device that allows for rapid availability of ammonia from a solid storage material by having a system with two main functionalities: A small, operational unit heated by e.g. electricity and a larger ammonia storage unit that is used as a source to carry out on-board resaturation of the small unit.
However, when the smaller unit is heated for releasing ammonia by desorption, the elevated temperature results in heat loss to the surroundings. To reduce the power consumption during operation (i.e. lower fuel penalty on a vehicle), it requires insulation of the heated unit. But this has a negative effect on the speed or resaturation from the larger unit because it is difficult to remove the absorption heat when the smaller unit is insulated.
Therefore, there is a need for a new method and device for improving performance both in terms of power consumption as well as rate of resaturation.